Orlando International Airport symbolizes the region's future: a skylit terminal, sleek jumbo jets, people in constant motion. It is the 17th busiest airport in the nation and growing.

The slough, 650 acres in all, acts as a kidney, cleaning water that runs through it, and eventually meanders hundreds of miles south in rivers, swales and lakes into the Everglades.

The airport, according to Orlando Mayor Bill Frederick, is akin to the heart of Central Florida's ever-revving ''economic engine.''

Slough and airport serve vital functions, but one apparently will have to yield to the other.

The slough, it seems, is on the planned site of the airport's fourth runway. If the 9,000-foot-long strip is built, the slough will die, buried under tons of concrete.

A large, intact wetlands area such as Bull Slough has never been taken by development in the state. Environmentalists do not want a precedent set.

Airport executives counter that they cannot build the runway elsewhere because of potential pollution problems from jet-engine exhaust. Moving could cost millions of dollars in added expense, including the purchase of part or all of the exclusive Lake Nona golf course to the southeast.

Can the needs of nature and the airport be balanced?

Environmentalists are skeptical; airport managers say yes.

Take out Bull Slough, Charles Lee of the Audubon Society said, and ''it would be another increment of degradation. It would not kill the system, but it would be a fairly big nail in the coffin, likely to splinter the wood.''

Orlando International director John Wyckoff scoffs at such suggestions.

Airport staffers, he said, are putting together a multimillion-dollar proposal that would replace Bull Slough with almost 4,000 acres, some in reclaimed wetlands nearby, and improve the quality of water running south from the airport.

That sounds good, environmentalists say. Give us proof.

''Right now they're just coming to us with a promise, and that's it,'' said Ed Edmundson, an analyst with the South Florida Water Management District.

Edmundson points to an airport document several inches thick that discusses the fourth runway in great detail but gives short shrift to offsetting the loss of Bull Slough.

''A detailed mitigation plan for impacts associated with the fourth runway . . . has not been completed,'' states the airport's development of regional impact statement, or DRI.

Edmundson's water management district, along with several other government bureaus, including the federal Environmental Protection Agency, is reviewing the airport's fourth runway plans. A no from one of the groups would kill the $80 million project.

Bull Slough aside, some government officials are not convinced that the fourth runway should be built.

''In my mind, I don't think they've really proven they need it,'' said Erin LeClair, a biologist with the EPA in Atlanta. Her office is checking the airport plan.

The airport opened a third runway Sept. 1 and is at 80 percent of capacity with 500 commercial flights a day, Wyckoff said.

Without a fourth runway by 1995, Wyckoff said a study by the Federal Aviation Administration estimated, Orlando International would lose $88 million a year in wasted fuel and lost man-hours because jets would have to wait before landing and taking off.

On average it costs $100 a minute to operate a jet while it idles on the ground.

The fourth runway would act mostly as a relief valve, accepting about 15 percent of the flights on any given day. Seventy percent of the traffic would use the middle two runways because they are closer to the main terminal.

It would be on what is now the eastern boundary of the airport, cutting through the northern part of the slough and running alongside it the rest of the way.

The runway center line would be in the middle of a 155-foot-wide corridor cleared years ago for a power line, which would be relocated at a cost of about $14 million to the airport.

Lee contends that the runway should be moved about 1,000 feet to the east to avoid the slough.

The EPA, under directions from President Bush to preserve wetlands whenever possible, is asking the airport to explore other options.

''They haven't really addressed any other alternatives other than no-build. . . . They've got a long way to go,'' LeClair said.

Without EPA approval, chances are poor that the airport would be able to get federal tax money to build the runway, said Jim Sheppard, the FAA's top administrator in Florida.

''It's up to them to solve the problem,'' Sheppard said. ''Historically we've been able to solve them. The airport is important to Central Florida and the nation.''